Replace interface{} with properly defined interfaces

- Add VirtualMachine interface with GetID(), GetActorID(), GetState()
- Add VMState type with idle/running/paused/stopped states
- Add RuntimeModel interface for event storming model contracts
- Add RuntimeMessage interface for actor message contracts
- Add VMProvider interface for decoupled VM access
- Update VMRegistry.GetActiveVMs() to return map[string]VirtualMachine
- Update Runtime interface to use RuntimeModel and RuntimeMessage
- Update DistributedVMRegistry to use VMProvider instead of interface{}
- Add SetVMProvider method to DistributedVM for dependency injection

This improves type safety, makes contracts explicit, and enables better
IDE support while avoiding import cycles through interface segregation.

Closes #37

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cluster/types.go

@@ -137,3 +137,43 @@ type LeadershipLease struct {
 	ExpiresAt time.Time `json:"expiresAt"`
 	StartedAt time.Time `json:"startedAt"`
 }
+
+// VirtualMachine defines the interface for a virtual machine instance.
+// This interface provides the minimal contract needed by the cluster package
+// to interact with VMs without creating import cycles with the runtime package.
+type VirtualMachine interface {
+	// GetID returns the unique identifier for this VM
+	GetID() string
+	// GetActorID returns the actor ID this VM represents
+	GetActorID() string
+	// GetState returns the current state of the VM
+	GetState() VMState
+}
+
+// VMState represents the state of a virtual machine
+type VMState string
+
+const (
+	VMStateIdle    VMState = "idle"
+	VMStateRunning VMState = "running"
+	VMStatePaused  VMState = "paused"
+	VMStateStopped VMState = "stopped"
+)
+
+// RuntimeModel defines the interface for an EventStorming model that can be loaded into a runtime.
+// This decouples the cluster package from the specific eventstorming package.
+type RuntimeModel interface {
+	// GetID returns the unique identifier for this model
+	GetID() string
+	// GetName returns the name of this model
+	GetName() string
+}
+
+// RuntimeMessage defines the interface for messages that can be sent through the runtime.
+// This provides type safety for inter-actor communication without creating import cycles.
+type RuntimeMessage interface {
+	// GetTargetActorID returns the ID of the actor this message is addressed to
+	GetTargetActorID() string
+	// GetType returns the message type identifier
+	GetType() string
+}